An ignition device of an engine stores magnetic energy in an ignition coil by supplying a primary current to a primary coil connected to a power source.
A secondary current generated in a secondary coil when the primary current is cut off is caused to flow in a gap formed between a center electrode and a ground electrode of a spark plug, thereby generating a plasma discharge in the gap of the spark plug.
At this time, it is possible to generate a plasma discharge (pulse discharge) effective for deodorization, sterilization, film formation, decomposition of harmful gas, ignition, etc. by applying a very high voltage pulse between the electrodes of the spark plug.
However, implementation of such a pulse discharge requires continuous supply of high voltage pulse at short cycles, and thus there is a problem that the supplied power is increased.
For this reason, it is disclosed in Japanese Patent No. 5,307,284 that a first pulse of high energy is applied until arc discharge (electrical breakdown) occurs between electrodes of a spark plug, and after the arc discharge has occurred between the electrodes of the spark plug, the arc discharge between the electrodes is maintained by applying a second pulse of low energy.
At this time, a peak voltage value of the second pulse is set to be lower than a peak voltage of the first pulse, and a pulse cycle of the second pulse is set to be shorter than a pulse cycle of the first pulse.
A pulse frequency of the second pulse should be high enough to maintain the arc discharge generated between the electrodes, and care should be taken so that power not be excessively consumed by making the pulse frequency too high.
By the second pulse thus adjusted being applied after the arc discharge has occurred between the electrodes of the spark plug, it is possible to reduce the power supply as well as to reduce the cost such as the running cost of the discharge device.
In Japanese Patent No. 5,307,284, for example, a case is supposed where the frequency of the first pulse is set to a frequency at which a voltage resonance in a circuit including the spark plug and the secondary coil becomes strongest (hereinafter referred to as a resonance frequency) or a frequency approximately equal thereto.
In this case, if the arc discharge occurs between the electrodes of the spark plug, impedance between the electrodes of the spark plug changes and the resonance frequency changes accordingly.
Therefore, if the first pulse is applied even after the arc discharge occurs, the secondary current flowing into the spark plug is insufficient because the voltage resonance does not occur or becomes weak, and there is a possibility that the arc discharge occurring in the spark plug to be in an unstable state.
In such a case, there is a possibility that the ignitability of the fuel deteriorates and the combustion state deteriorates.